This invention relates to material processing and more specifically to a method of providing pulses of laser radiation having pulse characteristics resulting in high absorption within a material.
Material processing by means of focused laser radiation is well known in the art. Materials, in particular metals, have been welded, cut, surface treated, drilled, etc., utilizing laser radiation in the pulsed and continuous wave modes with wavelengths varying from the ultraviolet to the infrared. The utilization of laser radiation with wavelengths in the infrared for welding and cutting of materials such as metals has been hindered by the high surface reflectivity of most metals to radiation having wavelengths in the infrared. Additionally, once the high surface reflectivity has been destroyed by the interaction of the radiation with the surface of the material, a plasma resulting from the melting and vaporization of the material is typically generated juxtaposed the surface along the path of the radiation. The plasma is typically highly absorptive of the radiation resulting in a reduction of the intensity of the radiation incident on the workpiece and a corresponding reduction in the efficiency of the material processing. For welding applications, the initial high reflectivity of metals can be significantly reduced and/or eliminated by establishment of the so-called deep penetration welding mode of operation well known in the art. In high reflectivity materials, such as aluminum alloys, copper, etc., the threshold power density for the destruction of the surface reflectivity is typically considerably above the level appropriate for establishing a uniform fusion zone. Thus the situation arises in which either nearly complete reflection of the incident radiation occurs or, upon the destruction of the surface reflectivity, excessive energy absorption occurs with resulting explosive boiling and porous weld formation.
Attempts to obviate this problem by initiating the welding process with a high energy beam to destroy the reflectivity followed by a reduction in the energy level to effect the weld have not been successful; reflection-dominated behavior reoccurs as energy level is decreased. Thus, the welding process will not continue if the energy in the beam is decreased below the threshold level. Morse in U.S. Pat. No. 3,588,440 filed June 26, 1969 discloses a laser combination energy system utilizing a first and second laser for material processing wherein the first laser is adapted to provide a high power pulse to effectively destroy surface reflectivity and the second laser is adapted to supply a continuous laser beam for maintaining the melt. Brown et al in U.S. Pat. No. 3,860,784 filed March 14, 1973 entitled "Deep Penetration Welding Using Lasers" and held with the present application by a common assignee discloses a method of utilizing a laser beam for deep penetration welding wherein the power in the beam is sufficiently high to overcome the relatively high reflectivity of a metallic workpiece. A molten zone generated by the interaction of the radiation and the workpiece is moved relative to the laser beam at a speed of at least ten focused spot diameters to allow the molten zone to become dynamically stable and translate through the material to form the weld. Once deep penetration welding is initiated effective beam absorption occurs.
Several methods are known in the art for suppressing the formation of a plasma juxtaposed the workpiece. Locke in U.S. Pat. No. 3,824,368 filed Jan. 11, 1973 discloses a method of welding in which a concentrated high power laser beam is moved along a workpiece surface with a flow of inert gas directed across the path of the beam adjacent the area where the radiation interacts with the workpiece to inhibit the formation of a beam absorbing plasma. The flow of gas suppresses the formation of the plasma and improves the metal working process by allowing the laser energy to pass uninhibited to the workpiece. In addition Banas et al in U.S. Pat. No. 4,000,392 filed Dec. 9, 1975 and Banas et al in U.S. Pat. No. 4,078,167 filed Feb. 9, 1977, both held with the present application by a common assignee, disclose welding devices adapted for use with a laser beam which are adapted for providing a stream of suppression gas across the path of a welding beam to suppress the formation of a beam absorbing plasma. The amount of gas flow is typically directly proportional to the power level of the laser beam. However, flowing a gas across the interaction zone for plasma suppression conflicts with the requirements of a quiescent weld zone for the generation of a smooth contamination free weld.